The heart of any ultrasound (imaging) system is the transducer which converts electrical energy in acoustic energy and back. Traditionally, these transducers are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so-called capacitive micro-machined ultrasound transducers (CMUT). A CMUT comprises a membrane (or diaphragm), a cavity underneath the membrane, and electrodes forming a capacitor. For receiving ultrasound waves, ultrasound waves cause the membrane to move or vibrate and the variation in capacitance between the electrodes can be detected. Thereby, the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the membrane to move or vibrate and thereby transmitting ultrasound waves.
However, charging is a known disadvantage of capacitive micro-machined ultrasound transducer. WO 2010/032156 A2 describes a capacitive micro-machined ultrasound transducer with a specific layer structure which solves the charging problem. A first isolation layer comprising a dielectric is arranged between the first electrode and the second electrode. Further, a second isolation layer comprising a dielectric can be arranged between the second electrode and the cavity. Especially, a so-called ONO (Oxide-Nitride-Oxide) dielectric layer renders a solution to charging.
In WO 2010/032156 A2, the first dielectric isolation layer and the second dielectric isolation layer electrically isolate the first electrode and the second electrode. Such dielectric isolation layers determine to a fair extent the overall performance of the CMUT device. In the ideal case, dielectric isolation layers are very thin or have high dielectric constant and a high breakdown voltage. However, an ONO dielectric layer has its limitations and can be deposited only at relative thick layers (e.g. about 250 nm using “Plasma-enhanced chemical vapor deposition” (PECVD)) and a low dielectric constant, as the dielectric constant of nitride is about 5 to 7. Thus, the performance of the CMUT is limited by a minimum thickness of the ONO dielectric layer, the electrical breakdown voltage and its dielectric constant. A particular problem with such a CMUT device can be that the operating voltage is rather high and the output pressure relatively low. Therefore, there is a need to further improve such a CMUT.